The overall goal of this proposal is to understand the mechanism of myeliod leukemogenesis by v-myb, the oncogene of avian myeloblastosis virus (AMV). This oncogene causes only hematopoietic malignancies and its protein product is one of a small group of nuclear oncogene products. A series of well- defined mutations will be introduced into v-myb using recombinant DNA technology. These mutated v-myb genes will then be introduced into infectious, independently selectable neo- myb proviruses. These proviruses can be transiently expressed in simian COS cells and continuously expressed in cloned QT6 quail fibroblasts and in BM-2 chicken myeblostasts. Correlation of transformation in vitro and in vivo by these mutant viruses with the structural and functional properties of their mutant p48 v- myb oncogene products will be used to address specific questions: 1. What is the minimum region of v-myb required for transformation? 2. What is the signal sequence for nuclear transport of p48 v- myb, and is such transport necessary for leukemogenesis? 3. Is the in vitro DNA binding activity of p48 v-myb related to its transforming capacity? 4. What specific intermolecular associations of p48 v-myb in vivo are required for transformation? 5. Does p48 v-myb regulate the expression of c-myb, which is generally not expressed in v-myb transformed cells? 6. Is p48 v-myb a general trans-activator of transcription as has been reported for the products of adenovirus E1A, c-myc, and HTLV-I and II X genes. 7. Is p48 v-myb required for ongoing DNA replication in intact cells and in isolated nuclei of AMV-transformed cells? These studies of v-myb appear to be particularly relevant to human leukemogenesis because the c-myb proto-oncogene is expressed at high levels in human leukemias, its expression is down-regulated during myeloid differentiation, it is amplified in certain human leukemic cell lines, and its chromosomal location suggests that it may be involved in specific translocations in human leukemias. In addition, c-myb is activated by retrovial insertion is a series of murine hematopoietic tumors.